Likely cause: the igniter or flame sensor: furnace clicking, won't ignite

Why this happens

Trane lists the igniter and flame sensor right after the gas supply as reasons a furnace won't ignite. One HVAC contractor, Coolray, explains that the click is the spark from a spark igniter, and that a sooty flame sensor or pilot problem can keep the burners from staying lit. Trane says you should not attempt to clean or replace the flame sensor yourself, and warns that a failed flame sensor can let gas build up.

What to do right now

Set the thermostat to Off so the furnace stops trying to light.
Look for a blinking light through the small window in the furnace door and count the blinks. Don't remove the door.
Don't open the furnace to clean, relight or adjust anything inside.
Write down the brand, model and serial from the furnace's data plate.
If you smell gas at any point, leave the house and call 911 or your gas utility from outside.

What to tell the HVAC technician

That the furnace clicks repeatedly but never lights, or lights and then goes out.
The blinking light pattern: color, number of blinks, and any pause.
That the thermostat, power, filter and gas handle all checked out.
The brand, model, serial and approximate age of the furnace.

Build the full list: what to tell the HVAC technician.

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References (every fact above comes from one of these pages)

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